Use of capacitance to measure relative position between two objects is widely known. One method of position sensing involves affixing two conductive plates to the objects that are to move relative to one another. The plates typically are secured to the objects so that they overlap and are parallel to each other, and are spaced by a gap. The two plates, together with an interposed dielectric (e.g., air) produce a capacitance which depends, in part, on the extent to which the plates overlap each other. As the objects move, the amount of overlap changes, resulting in a corresponding change in capacitance. From the capacitance change, the amount of relative displacement between the objects is determined.
One difficulty with the above approach is that various factors may contribute to a given change in capacitance. Assuming a rectilinear x-y-z coordinate system with a pair of conductive plates that are parallel to the x-y plane, capacitance variations will result from relative translation occurring between the plates along all three of the coordinate axes. However, many position sensing systems, including the simple system described above, cannot differentiate between a capacitance change resulting from motion occurring along one axis from a capacitive change due to motion occurring along another axis.
The above problem is of particular concern in certain types of micro electro-mechanical systems (MEMS), such as in very small computer storage devices. Some such storage devices include a storage medium that is designed to move within an x-y plane relative to an associated read/write device. To accurately access and write data to the medium, the exact relative position of the storage medium and read/write device must be known. The electrostatic drive mechanisms and other actuating mechanisms used with these devices typically are effective at producing x-y motion, however they sometimes produce incidental z-axis motion. The conventional capacitance position sensing system described above can produce erroneous position readings in the event of such incidental z-axis movement, for the reasons explained above. This may result in, among other things, the wrong data being read out, or in the accidental overwriting of existing data.
Another problem with existing capacitance-based position sensors is limited sensitivity. Particularly in micro storage devices and other MEMS systems, it desirable that the employed position sensor produce an output that varies significantly as a function of a given change in position. In some storage devices, position must be measured in fractions of a nanometer. Many existing sensors simply are not sensitive enough to provide output suitable for determining position to that fine a resolution.